118 



NEPHRITE AND JADEITE. 



n ,,., llrl ang i os f extinction were measured varying from no to 150. 



responds quite closely with a uephrite Inn,, the Belaja Ki ver, in Siberia, 

 LT described and figured by Beet and Muschketow * but it is more 

 ,„,,„,„ m text nre, and resembles more nearly the jade from He* 'Zea 

 hU(1 (S ee also figs. 1 and 3.) The Point Barrow specimen, .SOGoS, has 

 fcbe same structure a, B, and needs no further description. 



T||(1 fore „. oin g evidence is sufficient to show the essential identity oi 

 all the Uaskan jades, ami t » dispose of the theory that their presence 

 ',„ Alaska is t<. be accounted for upon the basis of trade with Siberia. 

 Thai theory is also negatived by the discovery, announced by Mr. G. M. 

 Dawson t o( small nephrite bowlders on the upper part of the Lewes 

 River not far from the eastern boundary of Alaska. But these neph- 

 ritesare also strikiugly like those from many other localities, and two 

 f the latter have been included in our comparisons. First, a water- 

 worn dark-green bowlder from New Zealand, sent to the Museum by 

 Sir Fulius ilaast : and second, a small implement from Robenhausen, 

 I a ke Pfaffikon, Switzerland, out of the collection of Mr. Thomas Wil- 

 son. The latter specimen, also green, had a specific gravity of 3.015, 

 as determined by Dr. Ballock. The analyses are as follows : 



i ^nii inn 



Silica 



Alumina 



Perrons oxide 







Magnesia . . . . 



New 

 Zealand. 



.83 



56. 71! 



:t. 22 



5.96 



13.24 



19. 4-.» 



99.40 



Swiss. 



.63 



56. 87 



1.50 



G. 33 



13.45 



21.06 



99.81 



All the iron is represented here as ferrous. Traces of manganese 

 were present, but alkalies were not looked for. 



In thin section, under the microscope, the Robenhausen jade is seei 

 to be made up of an extremely line and compact aggregate of fibers, 

 scales, and tufts, all arranged with their longer axes approximately 

 parallel, so as to produce a more or less schistose structure. Sections 

 cot parallel to this schistosity exhibit, in ordinary light, colorless, elon- 

 gated, narrow areas, bounded by very irregular wavy lines which a higl 

 power shows to be rifts stained by impurities. Between crossed nicoh 

 the entire field is converted into a confused mass of brilliantly polar 

 izing libers, from which no measurement of extinction or cleavage an 

 gles are obtainable. Cross sections show a peculiar feltdike ground 

 mass of colorless particles, polarizing only in light and dark colors, am 

 blending into one another without definite lines of demarcation whei 

 the stage is revolved. The held shows n umerous larger elongated ar eas 



* Uebcr Nephrit und seine Lagerstatteu, p. 12, Fig. 1. 

 t Science, April -JO, 1888, p. 186. 



